Optimizing Operation of DC-To-AC Power Converter

ABSTRACT

In one embodiment, a power converter system includes an input terminal for receiving a DC input voltage. The power converter system delivers AC power to a load at an output terminal. A transformer is coupled between the input terminal and the output terminal. The transformer has a first winding, a second winding, and a third winding. The output terminal is coupled to the second winding. A half-bridge circuit, coupled between the input terminal and the first winding of the transformer, includes a first switch and a second switch coupled at a common node. The first and second switches are operable to be turned on and off for causing current to flow in the transformer during operation of the power converter system. Circuitry is close coupled to the first winding of the transformer. The circuitry is operable to provide a current path for transformer magnetizing current and reflected load current when both the first and second switches of the half-bridge circuit are turned off, thereby preventing energy from being fed back to the half-bridge circuit.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of co-pending U.S. patentapplication Ser. No. 12/080,274, filed Apr. 2, 2008, entitled,“Optimizing Operation of DC-To-AC Power Converter,” the entirety ofwhich is incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to power conversion, and moreparticularly, to optimizing operation of a DC-to-AC power converter.

2. Description of Related Art

Power converters are essential for many modern electronic devices. Amongother capabilities, power converters can adjust voltage level downward(buck converter) or adjust voltage level upward (boost converter). Powerconverters may also convert from direct current (DC) power toalternating current (AC) power, or vice versa. Power converters aretypically implemented using one or more switching devices, such astransistors, which are turned on and off to deliver power to the outputof the converter. Control circuitry is provided to regulate the turningon and off of the switching devices, and thus, these converters areknown as “switching regulators” or “switching converters.” The powerconverters may also include one or more capacitors or inductors foralternately storing and outputting energy.

A DC-to-AC converter according to previously developed designs may beimplemented with switching devices connected in a half-bridgearrangement. The converter may employ one or more free-wheeling diodeswhich are coupled in parallel to the switching devices. Thefree-wheeling diodes provide an alternate path for current to flow ifthe switching devices are turned off. Such previously designed DC-to-ACconverter, however, could be problematic. For example, energy feedbackthrough the free-wheeling diode can cause an uncontrollable dead zone inthe operation of the converter. In the uncontrollable dead zone, theoutput voltage may not change in response to the switching—i.e., theoutput voltage is out of control.

FIG. 1 is an exemplary waveform diagram 100 for a power convertersystem, according to previously developed designs. Diagram 100 includeswaveform 102 representing the voltage of the drive or control signalapplied to the control terminal (e.g., gate) of a high-side switch inthe half-bridge arrangement, waveform 104 representing the voltage ofthe drive or control signal applied to the control terminal (e.g., gate)of a low-side switch in the half-bridge arrangement, waveform 106representing the voltage at a node between the high-side and low-sideswitches, and waveform 108 representing the voltage of the output ACsignal of the power converter system.

As shown in diagram 100, the uncontrollable dead zone appears in thewaveform 106 after the low-side switch is turned off and before thehigh-side switch is turned on. As further shown in diagram 100, hardswitching may occur as the low-side switch turns on. As a result of theuncontrollable dead zone and hard switching, the AC output voltage ofwaveform 108 does not have a perfect sinusoidal form.

SUMMARY OF THE INVENTION

Briefly, in some embodiments, the present invention provides circuitryand methods for DC-to-AC power converter having a half bridge topologyand transformer, and in some cases using pulse width modulation (PWM)control. The circuitry and methods may employ an auxiliary winding,which is close coupled to primary winding of the transformer, and twoswitches (e.g., MOSFETs) which are connected in series (e.g., as commonsource type). The two switches can short the auxiliary winding during adead zone when both switches of the half bridge topology are off, thusoffering a current path for transformer magnetizing current andreflected load current. This prevents energy from feeding back to the DCsource. As such, the circuitry and methods support or help to maintaincontrol of the voltage at the output of the power converter, resultingin a more ideal sinusoidal AC output waveform.

According to an embodiment of the present invention, a power convertersystem includes an input terminal for receiving a DC input voltage. Thepower converter system delivers AC power to a load at an outputterminal. A transformer is coupled between the input terminal and theoutput terminal. The transformer has a first winding, a second winding,and a third winding. The output terminal is coupled to the secondwinding. A half-bridge circuit, coupled between the input terminal andthe first winding of the transformer, includes a first switch and asecond switch coupled at a common node. The first and second switchesare operable to be turned on and off for causing current to flow in thetransformer during operation of the power converter system. A clampingcircuit is close coupled to the first winding of the transformer. Theclamping circuit operable to clamp the common node of the half-bridgecircuit, thereby controlling a voltage at the common node when both thefirst and second switches are turned off

According to another embodiment of the present invention, a powerconverter system includes an input terminal for receiving a DC inputvoltage. The power converter system delivers AC power to a load at anoutput terminal. A transformer is coupled between the input terminal andthe output terminal. The transformer has a first winding, a secondwinding, and a third winding. The output terminal is coupled to thesecond winding. A half-bridge circuit, coupled between the inputterminal and the first winding of the transformer, includes a firstswitch and a second switch coupled at a common node. The first andsecond switches are operable to be turned on and off for causing currentto flow in the transformer during operation of the power convertersystem. Circuitry is close coupled to the first winding of thetransformer. The circuitry is operable to provide a current path fortransformer magnetizing current and reflected load current when both thefirst and second switches of the half-bridge circuit are turned off,thereby preventing energy from being fed back to the half-bridgecircuit.

Important technical advantages of the present invention are readilyapparent to one skilled in the art from the following figures,descriptions, and claims.

BRIEF DESCRIPTION OF DRAWINGS

For a more complete understanding of the present invention and forfurther features and advantages, reference is now made to the followingdescription taken in conjunction with the accompanying drawings.

FIG. 1 is an exemplary waveform diagram for a power converter system.

FIG. 2 is a schematic diagram of an implementation of a power convertersystem, according to an embodiment of the invention.

FIG. 3 is an exemplary waveform diagram for the implementation of apower converter system shown in FIG. 2, according to an embodiment ofthe invention.

DETAILED DESCRIPTION

Embodiments of the present invention and their advantages are bestunderstood by referring to FIGS. 2 and 3 of the drawings. Like numeralsare used for like and corresponding parts of the various drawings.

FIG. 2 is a schematic diagram of an implementation of a power convertersystem 10, according to an embodiment of the invention. Power convertersystem 10 can convert a direct current (DC) power to an alternatingcurrent (AC) power, and thus, is a DC-to-AC converter. Power converter10 receives the DC power from a DC power source 6 at an input terminal.The power converter system 10 delivers AC power to a load at outputterminals Out1 and Out2.

As shown, power converter system 10 includes switches 12, 14,transformer 16, capacitors 18, 20 and clamping circuit 22.

Switches 12 and 14 are coupled to the input terminal for the DC powersource 6. As shown, switches 12 and 14 are connected at a switching node(A) in a half-bridge arrangement or circuit, with switch 12 being the“high-side” switch and switch 14 being the “low-side” switch. As usedherein, the terms “connected” or “coupled,” or any variant thereof,covers any connection or coupling, either direct or indirect, betweentwo or more elements. The high-side switch 12 may be connected betweenthe DC input voltage source and node A. Switch 12 is turned on to allowcurrent to charge the capacitor 18, and turned off for discharge of thecapacitor. The low-side switch 14 may be connected between the node Aand ground (GND). The low-side switch 14 is turned off during the chargecycle for capacitor 18, and turned on as the capacitor 18 discharges.Each of the two switches 12, 14 can be implemented with any suitabledevice, such as, for example, a metal-oxide-semiconductor field effecttransistor (MOSFET), an IGBT, a MOS-gated thyristor, or other suitablepower device. Each switch 12 and 14 has a control terminal (e.g., gate)to which a respective driving voltage or control signal (H_Drv andL_Drv) may be applied to turn the switch on or off. Control signalsH_Drv and L_Drv can provide, for example, pulse width modulated (PWM)control and may be generated by a controller (not shown).

Capacitor 18 is coupled at one end to node A, and coupled at the otherend to transformer 16. Capacitor 18 charges through switch 12, anddischarges through switch 14. The transformer 16 includes a primarywinding 24 and a secondary winding 26. The primary winding 24 isconnected to the capacitor 18. In one phase of operation for powerconverter system 10, current flows through primary winding 24 in onedirection—from capacitor 18 and out through ground. In another phase ofoperation, current flows through primary winding 24 in the oppositedirection—from ground and out through capacitor 18. The secondarywinding 26 is connected to capacitor 20 and output terminals Out1 andOut2. Current flow through the primary winding 24 causes energy to bestored in the transformer 16 and transferred to secondary winding 26.Current flows in one direction or the other through secondary winding26, depending on the direction of current flow in the primary winding24. Current flowing through secondary winding 26 charges and dischargescapacitor 20, and causes an AC power to be provided at output terminalsOut1 and Out2.

In operation of power converter system 10, dead zones are provided ormay occur after one of the switches 12 and 14 of the half-bridge circuitis turned off and before the other of the switches 12, 14 is turned on.During the dead zones, energy may be fed back from capacitor 18 to nodeA, which causes the voltage at node A to increase. With previous designsfor DC-to-AC converter, the increase of voltage at node A between theswitches of the half-bridge is not controlled, and as such, the outputvoltage of the converter may not be responsive to the switching. Thus,the output voltage of the DC-to-AC converter is out of control.

Embodiments of the present invention provide circuitry and methods tominimize or eliminate the uncontrollable dead zone caused by energy feedback in a DC-to-AC converter, thereby widening the effective range forduty control. Embodiments of the present invention improve the AC outputwaveform of the power converter system 10 and reduce harmonicdistortion. Embodiments of the present invention also provide, support,or help the switches of the half-bridge circuit to work at zero voltageswitching (ZVS) and zero current switching (ZCS) conditions, thuspreventing significant recovery current caused by the body diodes ofthose switches.

In one embodiment, as shown in FIG. 2, a clamping circuit 22 isprovided. Clamping circuit 22 may function to minimize or eliminate theuncontrollable dead zones. Clamping circuit 22 can clamp the voltage atthe common node A between switches 12 and 14 of the half-bridge so thatit does not rise uncontrollably during the dead zone. Clamping circuit22 may be implemented with switches 28 and 30 coupled to an auxiliarywinding 32 of transformer 10. Auxiliary winding 32 is closed coupled toprimary winding 16—i.e., the couple coefficient of the windings is closeto unity (1).

Switches 28 and 30 may be connected in series as common source type tothe auxiliary winding 32. Switches 28 and 30 short the auxiliary winding32 when switches 12, 14 of the half-bridge are turned off. Thus,switches 28 and 30 offer a current path for transformer magnetizingcurrent and reflected load current in the dead zone, thereby preventingthe energy feed back to source. Each of the switches 28, 30 can beimplemented with any suitable device, such as, for example, ametal-oxide-semiconductor field effect transistor (MOSFET), an IGBT, aMOS-gated thyristor, or other suitable power device. Each switch 12 and14 has a control terminal (e.g., gate) to which a respective drivingvoltage or control signal (AH_Drv and AL_Drv) may be applied to turn theswitch on or off. Control signals AH_Drv and AL_Drv may be generated bya controller (not shown).

Clamping circuit 22 also helps the two half-bridge switches 12, 14 towork under zero voltage switching (ZVS) and zero current switching (ZCS)conditions, thus preventing the huge recovery current caused by bodydiodes of switches 12 and 14.

In some embodiments, all or a portion of the components of powerconverter system 10 can be implemented on a single or multiplesemiconductor dies (commonly referred to as a “chip”) or discretecomponents. Each die is a monolithic structure formed from, for example,silicon or other suitable material. For implementations using multipledies or components, the dies and components can be assembled on aprinted circuit board (PCB) having various traces for conveying signalstherebetween.

The operation of power converter system 10 is described with referenceto FIG. 3. FIG. 3 is an exemplary timing diagram 300 for theimplementation of a power converter system shown in FIG. 2, according toan embodiment of the invention. Diagram 300 includes waveform 302representing the control signal H_Drv applied to the high-side switch 12in the half-bridge arrangement, and waveform 304 representing thecontrol signal L_Drv applied to the low-side switch 14 in thehalf-bridge arrangement, waveform 306 representing the voltage at a nodebetween the high-side and low-side switches, and waveform 308representing the voltage of the output AC signal of the power convertersystem. Superimposed over the waveform 302 for the H_Drv signal is awaveform 310 representing the control signal AH_Drv applied to theswitch 28 in the clamping circuit 22. Superimposed over waveform 304 forthe L_Drv signal is a waveform 312 representing the control signalAL_Drv applied to the switch 30 in the clamping circuit 22. In oneembodiment, a high value for any of control signals H_Drv, L_Drv,AH_Drv, and AL_Drv will cause the respective switch 12, 14, 28, and 30to turn on, whereas a low value for any of the control signals willcause the respective switch to turn off.

As shown in FIG. 3, the control signal AH_Drv for switch 28 iscomplementary to the control signal H_Drv for the high-side switch 12.Likewise, the control signal AL_Drv for switch 30 is complementary tothe control signal L_Drv for the low-side switch 14.

With reference to FIGS. 2 and 3, in an exemplary operation, whenhigh-side switch 12 is turned on by a high value for H_Drv controlsignal, current flows from the DC input source 6 to capacitor 18 andthrough primary winding 24 of the transformer 16. This causes current toflow through secondary winding 26 of the transformer 16, and the voltageat output terminals Out1 and Out2 rises. When high-side switch 12 isturned off, for example at time t1, current ceases to flow from the DCinput source 6 to capacitor 18. As such, current flow in the transformerdecreases, and the voltage at output terminals Out1 and Out2 decreases.When low-side switch 14 is turned on, for example at time t2, currentdischarges from capacitor 18 through switch 14. The current flowingthrough transformer 16 reverses and increases until time t3, whenlow-side switch 14 is turned off Thereafter, a new cycle begins.

During operation of power converter system 10, dead zones may occur whenboth the high-side switch 12 and the low-side switch 14 of thehalf-bridge arrangement are turned off More specifically, there is adead zone which occurs between the time when the low-side switch 12turns off and the high-side switch 12 turns on—e.g., for example,between time t3 and time t4. There is also a dead zone which occursbetween the time when the high-side switch 12 turns off and the low-sideswitch 14 turns on—e.g., between time t5 and time t6.

With previous designs of power converters, in such dead zone occurringbetween the time when the low-side switch turns off and the high-sideswitch turns on, energy could feed back through a free-wheeling diode,thus resulting in uncontrollable behavior of the power converter. Forthe dead zone occurring between the time when the high-side switch turnsoff and the low-side switch turns on, the voltage at the node betweenthe two switches in the half-bridge may rise such that the low-sideswitch turns on under non-ZVS and non-ZCS conditions, thus making forhard switching.

With embodiments of the present invention, switches 28 and 30 ofclamping circuit 22 are both turned on in the dead zones to short theauxiliary winding 32.

Thus, in the dead zone occurring between the time when the low-sideswitch turns off and the high-side switch turns on the dead zone (e.g.,between time t3 and time t4), auxiliary winding 32 and switches 28 and30 provide a path for transformer magnetizing current and reflected loadcurrent. In other words, between times t3 and t4, both of switches 12and 14 are off, and the primary winding 24 wants to feed back itscurrent to DC power source 6. But auxiliary winding 32 is shorted byauxiliary switches 28 and 30, so the voltage on primary winding 24 willbe zero. Primary winding 24 will not feed back current to DC powersource 6, and its original current is also transferred to auxiliarywinding 32 due to mutual induction. This prevents the energy intransformer 16 from feeding back through the half-bridge. As such,clamping circuit 22 functions to clamp the voltage at node A so that itdoes not rise uncontrollably during the dead zone. The output voltage ofpower converter system 10 is thus controllable, and responsive toswitching.

And in the dead zone occurring between the time when the high-sideswitch turns off and the low-side switch turns on (e.g., between time t5and time t6), clamping circuit 22 is an open circuit. The currentthrough auxiliary winding 32 ceases or is stopped, due to mutualinduction. As such, the voltage on primary winding 24 increases, and thevoltage at node A will decrease. This helps the switch 14 to work atzero voltage switching (ZVS) and zero current switching (ZCS). Thus,clamping circuit 22 provides for turn-on of the low-side switch 14 underZVS or ZCS conditions, thereby reducing or eliminating hard switching.ZVS and ZCS conditions on switch 14 eliminates the losses associatedwith diode reverse recovery current. This greatly reduces switchinglosses in power converter system 10.

By the operation of clamping circuit 22 described herein, embodiments ofthe present invention improve the AC output waveform from a DC-to-ACconverter and reduce the harmonic distortion. For example, referring toFIG. 3, the output waveform 308 of power converter system 10 has a moreideal sinusoidal form than that of power converters according toprevious designs.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions, andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims. That is, thediscussion included in this application is intended to serve as a basicdescription. It should be understood that the specific discussion maynot explicitly describe all embodiments possible; many alternatives areimplicit. It also may not fully explain the generic nature of theinvention and may not explicitly show how each feature or element canactually be representative of a broader function or of a great varietyof alternative or equivalent elements. Again, these are implicitlyincluded in this disclosure. Where the invention is described indevice-oriented terminology, each element of the device implicitlyperforms a function. Neither the description nor the terminology isintended to limit the scope of the claims.

1. A power converter system comprising: an input terminal for receivinga DC input voltage; an output terminal at which AC power is delivered toa load; a transformer coupled between the input terminal and the outputterminal, the transformer having a first winding, a second winding, anda third winding, wherein the output terminal is coupled to the secondwinding; a half-bridge circuit coupled between the input terminal andthe first winding of the transformer, the half-bridge circuit comprisinga first switch and a second switch coupled at a common node, the firstand second switches operable to be turned on and off for causing currentto flow in the transformer during operation of the power convertersystem; and a clamping circuit close coupled to the first winding of thetransformer, the clamping circuit operable to clamp the common node ofthe half-bridge circuit, thereby controlling a voltage at the commonnode when both the first and second switches are turned off.
 2. Thepower converter system of claim 1 wherein the clamping circuit isoperable to provide a current path for transformer magnetizing currentand reflected load current when both the first and second switches ofthe half-bridge circuit are turned off, thereby preventing energy frombeing fed back to the half-bridge circuit.
 3. The power converter systemof claim 1 wherein the clamping circuit comprises: a third winding ofthe transformer; and a third switch and a fourth switch coupled to thethird winding of the transformer, wherein the third and fourth switchesare operable to short the third winding when both the first and secondswitches of the half-bridge circuit are turned off.
 4. The powerconverter system of claim 3 wherein each of the third and fourthswitches of the clamping circuit comprises a MOSFET, and wherein thethird and fourth switches are coupled in series as common source type.5. The power converter system of claim 1 wherein the clamping circuit isoperable to support zero voltage switching (ZVS) and zero currentswitching (ZCS) conditions for at least one of the first and secondswitches of the half-bridge circuit during operation of the powerconverter system.
 6. The power converter system of claim 1 wherein theclamping circuit is operable to support maintaining control of a voltageat the output terminal during a dead zone in which both the first andsecond switches of the half-bridge circuit are turned off.
 7. The powerconverter system of claim 6 wherein the clamping circuit prevents energyfrom being fed back to the half-bridge circuit during the dead zone. 8.The power converter system of claim 1 wherein at least one of the firstand second switches of the half-bridge circuit is controlled by pulsewidth modulated (PWM) control.
 9. A power converter system comprising:an input terminal for receiving a DC input voltage; an output terminalat which AC power is delivered to a load; a transformer coupled betweenthe input terminal and the output terminal, the transformer having afirst winding, a second winding, and a third winding, wherein the outputterminal is coupled to the second winding; a half-bridge circuit coupledbetween the input terminal and the first winding of the transformer, thehalf-bridge circuit comprising a first switch and a second switchcoupled at a common node, the first and second switches operable to beturned on and off for causing current to flow in the transformer duringoperation of the power converter system; and circuitry close coupled tothe first winding of the transformer, the circuitry operable to providea current path for transformer magnetizing current and reflected loadcurrent when both the first and second switches of the half-bridgecircuit are turned off, thereby preventing energy from being fed back tothe half-bridge circuit.
 10. The power converter system of claim 9wherein the circuitry comprises: a third winding of the transformer; anda third switch and a fourth switch coupled to the third winding of thetransformer, wherein the third and fourth switches are operable to shortthe third winding when both the first and second switches of thehalf-bridge circuit are turned off.
 11. The power converter system ofclaim 10 wherein each of the third and fourth switches of the clampingcircuit comprises a MOSFET, and wherein the third and fourth switchesare coupled in series as common source type.
 12. The power convertersystem of claim 9 wherein the circuitry is operable to support zerovoltage switching (ZVS) and zero current switching (ZCS) conditions forat least one of the first and second switches of the half-bridge circuitduring operation of the power converter system.
 13. The power convertersystem of claim 9 wherein the circuitry is operable to supportmaintaining control of a voltage at the output terminal during a deadzone in which both the first and second switches of the half-bridgecircuit are turned off.
 14. The power converter system of claim 13wherein the circuitry prevents energy from being fed back to thehalf-bridge circuit during the dead zone.
 15. The power converter systemof claim 9 wherein at least one of the first and second switches of thehalf-bridge circuit is controlled by pulse width modulated (PWM)control.